In the use of variable displacement hydraulic pumps, it is desirable to vary the displacement of pump in response to a control in order to obtain maximum efficiency. It has heretofore been suggested that an electro hydraulic control system be provided. Typical such systems are shown in U.S. Pat. No. 4,139,987 wherein the system senses the load and varies the displacement.
Most prior art servo controls for controlling pump displacement through yoke positioning use a two-stage servo valve with a flapper nozzle arrangement as typified in U.S. Pat. No. 4,139,987. Such arrangements are costly to manufacture. Other two-stage servo valves used in this type application use a two spool arrangement but the small pilot spool stage has a low tolerance to contamination due to the small size.
Since single-stage servo valves are inherently more contamination tolerant and less costly than two-stage servo valves, they have been considered for use with the yoke-positioning controls of hydraulic pumps. However, such single-stage servo valves and pump combinations up to now have been limited to these applications which require relatively low flow rates and low speed response times, for example 500 milliseconds. In such applications requiring higher flow rates and faster response times, for example 70-100 milliseconds, single-stage servo valves become unstable due to the large flow forces action on the spool of the single-stage valve. For this reason the two-stage servo valve and pump combination predominate where higher flow rates and faster response times are required.
Among the objects of the present invention are to provide an electro hydraulic variable displacement pump control system which functions in response to actual movement of a movable element in the pump which controls displacement of the pump and functions to quickly and efficiently control the pump displacement.
In accordance with the invention, the variable displacement pump control system comprises a variable displacement pump having a movable element for controlling pump displacement, a hydraulic motor for moving the movable element, and a control module comprising a transducer for producing an electric signal corresponding to the actual position of the movable element, means for producing an electrical signal corresponding to the desired pump displacement, means for comparing the electrical signal from the transducer and the electrical signal corresponding to the desired displacement of the pump and producing an error signal, and a control operable in response to the error signal to meter fluid flow from the pump output to the hydraulic motor. The control comprises a relief valve, a compensator valve and a servo valve. The servo valve is preferably a single-stage valve that includes damping orifices to provide stability.